Radio frequency power devices are well known in the art. Such power devices may generate radio frequency outputs at a fixed frequency or at variable frequencies oscillating within a specified operating frequency band. In general, such radio frequency power devices include therewithin a hermetically sealed housing which is either evacuated, as in the case of a vacuum tube or a magnetron, or is filled with an inert gas. A variety of well known systems are utilized to generate an electromagnetic field within the hermetic housing with the current induced from such an electromagnetic field being directed to an output antenna.
One commonly used arrangement for generating such an electromagnetic field includes positioning an anode ring about a centrally disposed cathode within the hermetic housing, and then establishing a magnetic field around the anode. This is commonly done by positioning a pair of magnets on either side of the anode. In this manner, an electrical field is created between the cathode and anode, and a magnetic field is generated within the interaction space between the cathode and anode, thereby establishing an electromagnetic field at the anode. In this particular arrangement, the anode ring defines a cavity space radially inwardly thereof wherein the frequency of the electromagnetic field is established. In other arrangements, such as in a coaxial magnetron tube, the cavity space wherein the frequency of the electromagnetic field is defined may be located exterior to the anode.
In the above described arrangement, the frequency of the electromagnetic field is defined by the physical size and shape of the cavity as well as the conductive properties of the material utilized to form the cavity. One known cavity arrangement includes a plurality of tabs or vanes projecting radially inwardly from the inner surface of the anode ring, and the frequency of the electromagnetic field is determined by the conductive properties of the vanes, the size of the vanes, the spacing between the vanes, and the manner in which the vanes may be electrically interconnected at their radially inner edges. Thus, by adjusting these variables of the vane structure and thereby changing the overall volume of the cavity, any desired frequency may be pre-selected prior to construction of the radio frequency device. The manner in which the vane structure is constructed and altered so as to establish a desired fixed frequency is well known in the art and will not be discussed in any detail herein.
Once the shape and the volume of the frequency determining cavity is established, the frequency of the electromagnetic field generated within such a device is set, and the resultant frequency output of the device becomes fixed. In variable frequency power devices, however, the frequency of the electromagnetic field within the hermetic housing is tunable or varied in an oscillating manner by changing the volume of the frequency determining cavity in an oscillatory fashion, thereby changing the inductive properties thereof. One known technique for changing the volume of the frequency determining cavity includes positioning an electrically conductive member within the cavity and oscillating that member therewithin, thereby varying the volume of the cavity in an oscillatory manner. To achieve such oscillatory motion of an electrically conductive member within the cavity, prior devices have commonly utilized mechanical arrangements for moving the electrically conductive member.
One such mechanical tuning arrangement utilizes a thin wall bellows or diaphragm as part of the hermetic housing. The electrically conductive members are then mechanically connected to such a bellows or diaphragm, and the bellows or diaphragm are mechanically oscillated by a motor located outside the hermetic housing.
Another known tuning arrangement for changing the volume of the frequency determining cavity includes positioning electrically conductive members within the cavity and rotating such members along the inner surface of the anode ring. Such rotation is effected by magnetically coupling the rotating electrically conductive member to an electromagnetic power source disposed outside the housing. A distinct disadvantage to this latter technique, however, is that by rotating an electrically conductive member within the frequency determining cavity, the electromagnetic field frequency can be varied, but not in an oscillatory manner.
The movable bellows or diaphragm arrangement described above, however, also has certain disadvantages. One major disadvantage with this mechanical tuning arrangement is that the walls of the bellows or diaphragm must be relatively thin to effect such movement and are thereby subject to mechanical fatigue and failure. If such a bellows or diaphragm does fail, the vacuum or inert gas environment within the hermetic housing is destroyed, and the power source thereby becomes useless. Another disadvantage is that since the bellows or diaphragm must be constructed from a thin walled material, atmospheric gas can penetrate such thin material over a period of time and can thereby affect the internal environment. Therefore, such mechanical arrangements have a relatively short storage or shelf life.
A further disadvantage of the above mechanical tuning assemblies is that a significant energy input is required to operate such assemblies. This requirement is due to the mechanical resistance offered by the bellows or diaphragm arrangement as well as to the atmospheric dampening effect on the mechanical parts located exterior to the hermetic housing or envelope.
The novel tuning assembly of the present invention, however, overcomes the disadvantages of known mechanical tuning assemblies, and provides a relatively simple yet efficient means for oscillating the frequency of the electromagnetic field generated within such radio frequency power sources.
U.S. Pat. No. 4,223,246 to Osepchuk discloses an entirely different type of magnetron than is being disclosed and claimed in the present case. Osepchuk discloses a relatively low-powered magnetron device, apparently used for a microwave oven or other microwave heating applications. This device provides continuous waves of the order of 500 to 2000 watts. The present invention, by contrast to Osepchuk, is a high peak-power, pulsed magnetron for radar applications. The present device provides power outputs several orders of magnitude higher, namely, in the power range of 20,000 watts to more than a megawatt. Osepchuk's cathode is made of thoriated tungsten wire with a round cross section capable of output current in the milliampere range. The present invention, by contrast, has a highly efficient helical cathode which is machined (not made of wire), has a rectangular cross section, and is capable of output current of the order of amperes, not milliamperes. The helix of the present invention is tightly wound and presents an overall cylindrical configuration with a much larger emission surface. The emissivity of the cathode in the present invention is also greatly enhanced by the material of which it is made, namely, 82% dense porous tungsten impregnated with a mixture of barium and calcium aluminates.
U.S. Pat. No. 2,700,000 to Levi et al discloses a thermionic cathode which is indirectly heated by heater 3 or 7. This patent shows a cathode impregnated with an alkaline earth composition but does not show or suggest a directly heated cathode for use in a pulsed magnetron having considerably higher current and power outputs.
Neither of the above-mentioned patents to Osepchuk or Levi et al show or suggest the type of highly efficient cathode needed for a high peak-power pulsed magnetron tube, such as is disclosed and claimed in the present application. For one thing, neither patent contemplates the current and power outputs required in the present invention, namely of the order of amperes and megawatts, and therefore neither patent contemplates a need for the emission efficiency of the cathode disclosed and claimed in this application. Neither patent shows or suggests a directly heated cathode made highly efficient by being impregnated with a very effective mixture of barium and calcium aluminates. Morever, it should be understood that a pulsed magnetron has to oscillate at the beginning of a pulse. The particular combination of improved emissivity and more rapid warm up in this cathode allows a pulsed magnetron to start oscillating much faster than is true of any other pulsed magnetron known to the applicant.